SOLVED:
To fix the issue, I had to use channel Name, instead of channel ID as part of the initial iframe URL.
(My backend server now makes one extra call to RocketChat API, every time, to get current channel Name.)
failing example:
<iframe src="https://rocket.example.com/channel/669fbc5eb19c62f47251fb7b" ... />
what works:
<iframe src="https://rocket.example.com/channel/JohnDoeChannelName" ... />
I would imagine using ID is preferred, as they are constant, and Name could potentially be changed.
RocketChat rewrites these URL’s seamlessly, and it works correctly in normal browser window, - but within iframe, something fails.
I’m not sure if this is even a bug, and if so, it may be version-related. We’re currently on 6.10